A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow organ wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, various types of hearing prostheses have been developed. For example, when hearing impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue within the cochlea 104 with small currents delivered by multiple electrode contacts distributed along the electrode. FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides audio information to an external signal processor 111 where various signal processing schemes can be implemented. The processed data communications signal with the audio information is then converted into a digital data format, such as a sequence of data frames, for transcutaneous transmission by an external transmitting coil 107 to a corresponding receiving coil in an implant processor 108. Besides extracting the audio information from the data communications signal, the implant processor 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
When hearing impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical vibration to the auditory system. With conventional hearing aids, a microphone detects sound which is amplified and transmitted in the form of acoustical energy by a speaker or another type of transducer into the middle ear 103 by way of the tympanic membrane 102. Interaction between the microphone and the speaker can sometimes cause an annoying and painful a high-pitched feedback whistle. The amplified sound produced by conventional hearing aids also normally includes a significant amount of distortion.
Efforts have been made to eliminate the feedback and distortion problems using middle ear implants that employ electromagnetic transducers. A coil winding is held stationary by attachment to a non-vibrating structure within the middle ear 103 and microphone signal current is delivered to the coil winding to generate an electromagnetic field. A magnet is attached to an ossicle within the middle ear 103 so that the magnetic field of the magnet interacts with the magnetic field of the coil. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear 103. See U.S. Pat. No. 6,190,305, which is incorporated herein by reference.
Middle ear implants using electromagnetic transducers can present some problems. Many are installed using complex surgical procedures which present the usual risks associated with major surgery and which also require disarticulating (disconnecting) one or more of the bones of the middle ear 103. Disarticulation deprives the patient of any residual hearing he or she may have had prior to surgery, placing the patient in a worsened position if the implanted device is later found to be ineffective in improving the patient's hearing.
U.S. Patent Publication 20070191673 and U. S. Provisional Patent Application 61/121,399, filed Dec. 10, 2008, which are incorporated herein by reference, describe driving a relatively large inertial mass to vibrate the skull bone of a hearing impaired patient. As shown in FIG. 2, a floating mass transducer (FMT) 203 is mechanically connected to the temporal bone of the patient. The mass of the floating mass transducer (FMT) 203 vibrates in response to the audio information in a data communications signal originating from an external processor 201 and transmitted to an implanted receiving coil 202. Bone conduction of the FMT vibrations through the temporal bone are transduced into fluid motion within the cochlea and perceived as sound.